Group By in MongoDB(with $group)

MongoDB provides a rich set of aggregation. $group is one of them, with the help of $group, we can group documents (similar as group By in SQL). Lets understand by an example:

Suppose we have following collection of blogs:
[js]
{ userId:1, name:”Suroor”, title:”AngularJS: Getting started with Directive”, rating:3}
{ userId:2, name:”Amit”, title:”AngularJS: Getting started with Directive”, rating:5}
{ userId:3, name:”PI”, title:”Prototype in Javascript”, rating:4}
{ userId:2, name:”Amit”, title:”this keyword in Javascript”, rating:5}
[/js]
And now we want to group all the blogs by userId and rating greater then 3:
[js]
db.blogs.aggregate([{$match:{rating:{$gt:3}}}, {$group:{ _id: ‘$userId’, title: { $push: ‘$title’}}}]);
[/js]
This will return grouped result of all the blogs with id and title whose rating is greater then 3. Here $match will match the condition, $group groups all the documents by field-name which is provided as _id and $push will add the title into result.

OUTPUT:
[js]
{
“result” : [
{
“_id” : 3,
“title” : [
“Prototype in Javascript”
]
},
{
“_id” : 2,
“title” : [
“AngularJS: Getting started with Directive”,
“this keyword in Javascript”
]
}
],
“ok” : 1
}
[/js]

Amit Kumar
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